Electric discharge gas lasers are well known and have been available since soon after lasers were invented in the 1960s. A high voltage discharge between two electrodes excites a gaseous gain medium. A resonance cavity containing the gain medium permits stimulated amplification of light which is then extracted from the cavity in the form of a laser beam. Many of these electric discharge gas lasers are operated in a pulse mode.
Excimer lasers are a particular type of electric gas discharge laser and have been known as such since the mid 1970s. A description of excimer lasers, useful for integrated circuit lithography, are described in U.S. Pat. No. 5,023,884 issued Jun. 11, 1991 entitled xe2x80x9cCompact Excimer Laserxe2x80x9d and U.S. Pat. No. 5,991,324 issued Nov. 23, 1999 entitled xe2x80x9cReliable, Modular, Production Quality Narrow-Band KrF Excimer Laserxe2x80x9d. Both of these patents have been assigned to Applicants"" employer, and these patents are hereby incorporated herein by reference. The excimer lasers described in the above patents are high repetition rate pulse lasers. In FIG. 1, the principal elements of the laser 10 are shown (FIG. 1 corresponds to FIG. 1 in Patent ""884). The discharges 22 are between two long (about 23 inches) electrodes 18 and 20 spaced apart by about ⅝ inch. Repetition rates of prior art lasers, like the ones described, are typically within the range of about 100 to 2000 pulses per second. These high repetition rate lasers are usually provided with a gas circulation system. In the above referred to laser, this is done with a long squirrel-cage type fan 46, having blades 48 as shown in FIG. 1 and in FIG. 2 which is FIG. 7 in Patent ""884. The fan blade structure is slightly longer than the electrodes 18 and 20 and provides sufficient circulation so that at pulse rates between 100 to 2000 Hz, the discharge disturbed gas between the electrodes is cleared between pulses. The gas used in the laser contains fluorine which is extremely reactive. The fan rotor driving fan shaft 130 is sealed, within the same environmental system provided by housing structure members 12 and 14, by sealing member 136 as explained at column 9, line 45 of Patent ""884, and the motor stator 140 is outside sealing member 136 and thus protected from the corrosive action of the fluorine gas. Heat in the gas which is produced by the electric discharge and the rapid circulation of the gas is removed by finned, water-cooled heat exchanger 58. An important use of these lasers is as a light source for integrated circuit lithography. The nominal output wavelength of these lasers is determined by the gas mixture. A KrF excimer laser operates at about 248 nm; an ArF excimer laser operates at about 193 nm and an F2 excimer laser operates at about 157 nm.
Electric discharge gas lasers of the type described in U.S. Pat. No. 5,023,884 utilize an electric pulse power system shown in FIG. 3 to produce the electrical discharges, between the two electrodes. In such prior art systems, a direct current power supply 22 charges a capacitor bank called xe2x80x9cthe charging capacitorxe2x80x9d or xe2x80x9cC0xe2x80x9d 42 to a predetermined and controlled voltage called the xe2x80x9ccharging voltagexe2x80x9d for each pulse. The magnitude of this charging voltage may be in the range of about 500 to 1000 volts. After C0 has been charged to the predetermined voltage, a solid state switch 46 is closed allowing the electrical energy stored on C0 to ring very quickly through a series of magnetic compression circuits comprising capacitor banks 52, 62 and 82 and inductors 48, 54 and 64 and a voltage transformer 56 to produce high voltage electrical potential in the range of about 16,000 volts across the electrode which produces the discharge which lasts about 50 ns.
In prior art lithography laser systems, the time between the closing of the solid state switch and the discharge is in the range of about 5 microseconds; however, the charging of C0 accurately to the pre-selected voltage has in the past required about 400 microseconds which was quick enough for pulse repetition rates of less than about 2,000 Hz. The reader should understand that accurate charging of C0 is very important since the control of the voltage level on C0 is in these systems the only practical control the laser operator has on the discharge voltage which in turn is the primary determiner of laser pulse energy. For laser light sources used for integrated circuit fabrication the precise timing of the pulses has not been critically important since for both stepper machines and scanning machines target areas on the wafer are illuminated with a number of pulses such as about 20 to 40 pulses during an interval of a few milliseconds.
Reticles used for integrated circuit lithography contain the patterns to be applied to the silicon wafer as a part of the process to create the integrated circuit. The pattern on the reticle is typically 3 or 4 times larger than the corresponding image on the wafer. Nevertheless, the dimension on the reticle are still very small, i.e., a few hundreds of nanometers. These patterns on the reticles typically in the past have been created with electron beams, and both reticles and wafers typically have been inspected with visible light microscopes.
What is needed are excimer laser systems optimized for reticle creation and inspection of both reticles and wafers.
The present invention provides a high repetition rate, compact, modular gas discharge, ultraviolet laser. The laser is useful as a light source for very rapid inspections of wafers in an integrated circuit fabrication process. It is also useful for reticle writing at very rapid rates. A preferred embodiment operates at pulse repetition rates of 1000 to 4000 Hz and is designed for round-the-clock production line operation. This preferred embodiment comprises a pulse control unit which controls the timing of pulses to an accuracy of less than 4 nanoseconds. Preferred embodiments of this gas discharge laser can be configured to operate with a KrF gas mixture, an ArF gas mixture or an F2 gas mixture, each with an approximate buffer gas, producing 248 nm, 197 nm or 157 nm ultraviolet light pulses.